This invention relates to telescope optics, and more particularly to an objective lens system for use with wavelengths in the visible spectrum and having an aplanat objective lens group as an objective lens component.
An objective lens system, as the term is used herein, is a lens systems which has two or more lens groups with each lens group having at least one lens. Each lens groups provides a particular function within the objective lens system.
A group of equations have been developed for the design of lens systems, including objective lens systems, and the lens groups therein. These equations are known as the "thin lens" equations Kingslake, Lens Design Fundamentals, Ch. 4, (1978); MIL-HDBK-141. ] These equations are: EQU .PHI.=.SIGMA..phi..sub.i ( 1) EQU .phi..sub.i =(n.sub.i -1)*cv.sub.i ( 2) ##EQU1## where .PHI. is the power of a lens group in the optical system, .PHI..sub.i is the power of an individual lens, n.sub.i is the index of refraction of an individual lens, cv.sub.i is the total curvature of an individual lens, and V.sub.i is the Abbe-V number, discussed below, of the glass of an individual lens. Equation (3) is the primary color correction equation and is used to correct for longitudinal chromatic aberration. Another equation, Equation 16 in MIL-HDBK-141, 11.4.3, is often utilized for correction of secondary color spectrum based on the partial dispersion of the optical glasses of the lenses.
Lenses are fabricated from optical glass. The refractive index of any optical glass or material changes with wavelength. This change, commonly known as dispersion, causes longitudinal chromatic aberration. That is, the focal length of the lens changes with wavelength.
Primary color is an indication of the variation in focal length at two specific wavelengths at the longest and shortest colors used. Since most lenses of optical glasses are used in the visible spectrum by a human observer, the standard convention is to measure primary color at the C-spectral line (a wavelength of 656.27 nm) and the F-spectral line (a wavelength of 486.13 nm). Depending on the exact nature of optical glasses, the focal length of a lens will reach an extreme somewhere within the stated range. If the design of a lens is such that the primary color is zero, the secondary color is a measure of the extent of chromatic aberration over the spectral range of use.
The Abbe-V number is a standard parameter of optical glass which is a function of the indices of refraction of a particular glass at certain wavelength. That is ##EQU2## where V.sub.d is the Abbe-V number and n.sub.d, n.sub.f, and n.sub.c are the indices of refraction of the glass at certain wavelengths, i.e, certain colors in the hydrogen emission spectrum. (C=656.27 nm, F=486.13 nm, and d=587.56 nm)
The primary color correction equation, Equation (3), discussed above, is used to set the optical power of the glass used in a lens at the C and F wavelengths. It ensures that the optical power of the glass at the C and F wavelengths is the same so that the image along the longitudinal axis is equal, i.e, at the same place, for these two colors.
Heretofore, glasses in an optical system have commonly been selected by using the Schwartzchild measure equation. This equation, which is the combination of Equations (1) and (2) above rewritten in a slightly different form for a unit focal length, provides ##EQU3## where n.sub.i and n.sub.i ' are the indices of refraction before and after the radius of curvature r.sub.i, and the height of incidence of the parallel auxiliary ray is denoted with h.sub.i, and the index n.sub.i is the surface-number from the first to the rear radius of the system. As implied by the Schwartzchild measure equation, the power of a typical designed lens group is dependent" on the indices of refraction of the materials selected and their curvatures. Since the index of refraction increases at longer wavelengths, it is known that the ideal case is to match their rate of change based on a knowledge of the relative partial dispersion. Alternatively, the primary color correction equation, Equation (3) above, is used to select the glasses. The primary color correction equation, however, expresses essentially the same relationships as the Schwartzchild measure equation but in a different form.
In any event, the approach taken in selecting the glasses in a collimator objective lens system using either the Schwartzchild measure equation or the primary color correction equation assumes that each separate lens group is designed so that chromatic aberrations introduced by one lens within the lens group are corrected by other lenses within that same group. That is, it is assumed that no significant chromatic aberrations are introduced by any lens group that must be corrected by a subsequent lens group.
Collimator objective lens systems are critical parts of larger optical systems such as are used on satellites. Collimators take light from the converging image from the reflective telescope typically used in such optical systems and translate it into parallel wavelengths for distribution to an optical sensor. Without a collimator, the telescope must image directly on the optical sensor. This is impractical in multichannel systems which have an optical sensor for each channel.
Severe packaging constraints are imposed on optical systems used in satellites. For obvious reasons, it is desirable that the optical system used in a satellite be as compact as possible. Principally for this reason, refractive collimators are more desirable than reflective collimators. The most compact optical system can be achieved by using a refractive collimator with a reflective telescope. By refractive collimator, it is meant that refractive lenses are used to make up the collimator.
A refractive collimator typically consists of two or more lens groups. These combined lens groups are designed to correct various well known optical aberrations to provide good optical imaging.
A problem with refractive collimators of the type heretofore used is that alignment is crucial, particularly as the field of view increases. Misalignment of the lenses which make up the refractive collimator causes optical aberrations such as coma, spherical aberration, and astigmatism to increase. If the misalignment is severe enough, the aberrations become large enough that the image is lost. This problem is exacerbated as the field of view increases. Optical aberrations are a function of the field of view and in some cases are powers of the field of view. Consequently, as the field of view increases, so too do the magnitude of optical aberrations. Thus, in systems having a wide field of view, even slight misalignments of the collimator lenses can cause sufficient aberrations so that the image is lost.
Aplanat lenses, which are refractive lenses, are known to be free from the optical aberrations of coma, spherical aberration, and astigmatism. Aplanats produce image magnification but do not produce collimated light. The image magnification produced by the aplanat objective lens permits the power of the other lenses in the collimator to be reduced. This allows for a longer focal length. It would therefore be advantageous to construct a refractive collimator of two or more lens groups But in this case, the first group is the objective lens group which magnifies the image and the second lens group, as well as any subsequent lens groups, collimates the magnified image and corrects for any optical aberrations introduced by the lens groups of the collimator. The longer focal length of this objective lens group permits easier correction of the optical aberrations introduced by individual lenses in the second lens group. Thus, the design of the second lens group is simplified.
Aplanats have been used as the objective lens in a lens system, including a collimator for use in the visible spectrum. In one collimator, a single element aplanat is coupled with an achromatic doublet to form a collimator. However, in this system, the aplanat is assumed not to introduce any chromatic aberrations. This simplistic approach ignores the most basic lens design theory. In another collimator, the first lens group comprises a doublet where one aplanat lens is coupled with another lens to correct any chromatic aberrations and also simulate an aplanat. The technique used to select the glasses in the lens groups of these two collimators is consistent with the standard approach discussed above. That is, each lens group is assumed to produce no chromatic aberrations that must be corrected by a subsequent lens group.
However, aplanats made from optical glasses have significant dispersion in the shorter wavelengths of the visible spectrum although not in the infrared. In other words, aplanats exhibit significant chromatic aberration in the visible spectrum and require significant color correction in the visible spectrum. Aplanats have not seen significant usage in collimators for the visible spectrum due to this chromatic aberration problem. The aplanatic "doublet" discussed above has not proven to be particularly desirable as the objective lens in a visible spectrum collimator since at least three glasses are typically required for the correction of secondary color spectrum. On the other hand, Germanium aplanats have been used as the objective lens in collimators for infrared where they do not exhibit chromatic aberration and thus do not require color correction.
It is an object of this invention to provide a collimator objective lens system for use in the visible spectrum which has one or more single element aplanat lenses making up the objective lens group wherein chromatic aberrations introduced by those the single element aplanat lenses are corrected by the second lens group in the lens system.
It is another object of this invention to provide a visible spectrum collimator having single element aplanat lenses as the objective lens group wherein chromatic aberrations are corrected by a second lens group and the lenses in the second lens group are so designed.
It is another object of this invention to provide a method for selecting the powers and glasses of lenses in a second lens group of a lens system which has one or more single element aplanats as the first lens group to correct the chromatic aberrations produced by the first lens group.